This invention relates to reflectors for reflecting radar signals so that they return substantially parallel to their angle of incidence. The simplest of these is the well known octahedral reflector which comprises three sheets of electrically conducting material arranged mutually orthogonally to one another and intersecting one another to provide eight trihedral re-entrant corners having a common geometric origin. Each trihedral re-entrant corner reflects a radar beam which enters that corner reasonably close to its axis at substantially its incident angle. When the incident radar signal arrives at an angle with respect to the axis of the trihedral re-entrant corner reflector the magnitude of the signal reflected back along the path of the incident signal falls off rapidly and falls off even more rapidly at incident angles greater than 20.degree.. With a standard octahedral reflector there are only eight trihedral corners attempting to cover the entire 4.pi. solid angle and, consequently there are large regions over which no effective reflection takes place from such an octahedral reflector even allowing for the contribution made by the dihedral reflecting components and the flat plates individually.
In an attempt to overcome the limitations of this conventional device, radar reflectors consisting of a single hand or double handed helical array of trihedral re-entrant corner reflecting elements have been produced. Such radar reflectors are illustrated in, for example, GB-A No. 681666 and EP-A No. 0000447, respectively. In such arrays the origins of the trihedral reflecting elements are located around a cylinder. Such reflectors are generally successful particularly for use on sailing boats in which they can be hoisted high into the rigging and so hoisted to a considerable height above the surface of the sea. They have also been used for other marine purposes such as navigational buoys. One of the difficulties encountered with conventional reflectors is being able to discriminate the echo from the reflector from a high background clutter. Another problem is caused by sea surface specular reflection as a result of a calm sea or ice. In this case phase cancellation occurs between the incident radar signal arriving directly from the transmitter and a radar signal which is reflected from the surface of the sea before its arrival at the reflector. These signals are separated by only a very small angle when the reflector is close to the sea surface. When such signals are presented to a single reflective corner, dihedral, or flat plate, no signal is returned since they cancel one another out. To some extent this effect can be overcome by raising the height of such a radar reflector from the surface of the sea.
It is also known from GB-A-1468516 to mount a trihedral re-entrant radar reflector assembly inside a cylindrical housing including wind vanes and mounted for rotation so that, in use, the reflector is rotated around an upright axis by the wind.